Optical fiber bragg grating (FBG) is commonly applied in various components for manufacturing dense wavelength division multiplexing (DWDM), such as FBG stabilizing laser source, and various DWDM devices used in multiplexer, de-multiplexer, and optical add-drop multiplexer (OADM). However, in actual applications, increment of environmental temperature may affect the performance of the FBG. Because the grating space and index of refraction of the FBG determine the central frequency of the reflected light, special care must be given to ensure the precision of the FBG. Since increment of environmental temperature will change the index of refraction of the FBG, thereby causing increment of the wavelength of the optical fiber thereby deviating from the designated central wavelength, measures shall be taken to prevent occurrences of such changes.
To resolve the above problem, U.S. Pat. No. 5,042,898 discloses a temperature compensated FBG device. The device comprises two metals with different thermal expansion coefficients. Relevant references further include, such as U.S. Pat. No. 5,703,978 and Applied Optics., Vol. 34 (30), p.6859, 1995 (G. W. Yoffe et. al.). However, the prior art still has the drawbacks of uneasily attaining the desired precision, being complicated in structures, involving difficult preparation steps, and being higher in cost. Therefore, it would be highly desirable to have an easily fabricated and simple temperature compensated FBG device with excellent temperature compensated result. For example, the use of materials with negative coefficient of thermal expansion (negative expansion materials) is one of the approaches and has been disclosed in U.S. Pat. No. 5,694,503, which is incorporated herein for reference.
Most materials expand upon heating and shrink upon cooling, whereas few materials shrink upon heating. Zirconium tungstate is a known isotropic negative expansion material within a temperature range from 0.3 K to its decomposition temperature of about 1050 K. This material was first synthesized by J. Graham et al. (J. Am. Ceram. Soc., 42 [11] 570, 1959) in 1959 and its negative expansion property was discovered in 1968 (J. Am. Ceram. Soc., 51 [4] 227, 1968).
According to the phase diagram of ZrO2 and WO3 (J. Am. Ceram. Sco., 50 [4] 211, 1967), ZrW2O8 is formed from ZrO2 and WO3 at 1105° C., and melts at 1257° C.; apparently it is only thermodynamically stable within a temperature range of about 150° C. It must be quenched (rapidly cooled) from high temperature to avoid decomposition into ZrO2 and WO3. Once formed, nevertheless, ZrW2O8 has a high degree kinetic stability, in metastable condition, below 770° C. (1050 K). Thus, ZrW2O8 will decompose into ZrO2 and WO3 when heated to about 770° C., and react to reform ZrW2O8 if the temperature is increased to 1105° C.
Generally, the preparation of ceramic body includes forming ceramic powders by the solid-state reaction or chemical synthesis process, grinding the powders, and compaction the grinded powders and sintering. Specifically, the preparation of single phase ceramic powders is followed by the sintering densification of the powders. In relevant researches, the zirconium tungstate ceramic body is normally prepared by using the above solid-state reaction process or chemical synthesis process.
In the earlier time, the ceramic powders were prepared by solid-state reaction, and then sintered to provide zirconium tungstate ceramic bodies. For example, according to the process described in Solid State Comm., 114, 453, 2000 (Yamamura et al.), weighted with appropriate ratio and mixed ZrO2 and WO3 powders were compacted and then calcined at 1473K for 12 hours in air to carry out the solid-state reaction to form zirconium tungstate, which was then rapidly cooled down to room temperature. After grinding the pellets, the resulting powders were compacted again and then sintered at 1473K for 12 hours for densification and quenched in liquid nitrogen to form a single-phase zirconium tungstate ceramic body. Nonetheless, the solid-state sintering process for the preparation of zirconium tungstate ceramic bodies normally requires ten or more hours to provide a pure phase zirconium tungstate ceramic bodies. Furthermore, if the particle sizes of the raw material powders are inappropriate or the admixing is unwell, it is difficult to obtain a uniform and single phase zirconium tungstate ceramic body by solid-state reaction process. The applicability of the zirconium tungstate ceramic body prepared thereby will thus be affected.
In addition to the solid-sate reaction process to prepare the zirconium tungstate ceramic powder, Sleight et al. proposed in 1996 the preparation of pure phase zirconium tungstate by a chemical synthesis process. Relevant publications, such as Science, 272, 90, 1996, Annu. Rev. Mater. Sci., 28, 29, 1998, J. Solid State Chem., 139, 424, 1998, and U.S. Pat. No. 5,514,360, are incorporated herewith for reference. The chemical synthesis process for the preparation of zirconium tungstate powder comprises the following steps. A solution containing Zr4+ and W6+ ions is heated to evaporate the liquid phase and thereby produce precipitate. The precipitate is heated to provide a mixture comprising ZrO2 and WO3 or ZrW2O8. The mixture is then grinded and re-heated to obtain a single-phase zirconium tungstate. It has been proved that the chemical synthesis process provides an efficient method to control the particle sizes of powders and the admixing. However, the process needs a solvent to adjust the pH value of the solution to obtain the precipitate of Zr4+ and W6+ and also requires another step of heating the precipitate. The preparation time is long and the preparation steps are complicated.
Surprisingly, the inventors of the present invention has found that when preparing zirconium tungstate by the reactive sintering process, the addition of powders of zirconium tungstate single crystal in the powders of raw materials comprising the Zr-containing compound and W-containing compound as the seeds for the formation of the grain of zirconium tungstate can reduce the formation energy and effectively simplify the procedure, shorten preparation time and save cost while obtaining zirconium tungstate ceramic bodies with uniform microstructure. In addition, by the formation of a second phase different from zirconium tungstate inside the ceramic body, the present invention efficiently provides the ceramic body with a desired expansion coefficient to provide the desired temperature compensation effect on FBG.